This invention relates to adhesives and adhesive compositions, particularly to adhesive compositions containing microspheres and more particularly to printable adhesive compositions containing microspheres and methods thereof.
A variety of methods to deliver printable or patternable adhesives exist and offer several advantages over conventional flood coating. These advantages include selective placement of adhesive on the substrate that results in reduced waste and improved handling. Each printing method requires that the printable material that is used possess particular rheological characteristics.
The following table describes a range of viscosities and characteristics of printable material classified by printing method.
Printing methods can be classified into two general fields, contact (impression) and non-contact (impactless) printing. Contact printing describes the processes in which an image is transferred through direct contact between the printing plate or image carrier and the substrate. Contact printing methods include lithography, gravure, flexography, and screen printing. In contrast, non-contact printing methods describe processes in which little or no contact is made with the substrate. Non-contact printing methods include electrophotography, thermal imaging, jet printing, and electrographic methods.
Screen-printing is a popular contact printing method due to the relative simplicity of both the printing process and equipment and the variety of surfaces that can be imprinted using this method. The screen printing process first involves making a stencil on a screen mesh that defines the pattern, text, image, etc. that is to be printed on a particular surface. Once the screen is prepared, a fluid (usually ink) is squeezed through the open areas of the stenciled mesh to transfer the pattern or image to this surface. As a final step, the fluid must be dried or otherwise fixed to prevent the distortion and preserve the integrity of the printed pattern or image.
Screen printing is unique among other printing methods as it can be used to deliver relatively thick fluid films, anywhere from 8 micrometers to over 30 micrometers in thickness. This thick coating capability renders screen printing an ideal method for coating discrete patterns of adhesives. Adhesive materials, particularly pressure sensitive adhesives, generally exhibit their desirable properties of high internal strength and clean removability from a adherend only if coated or printed at a sufficient thickness.
Screen printable fluids, including adhesives, must possess a particular balance of rheological properties that permits both sufficient fluid flow when squeezed through the screen mesh during the printing process and adequate resistance to flow to prevent smearing or soak through of the printed pattern. Such rheological characteristics can be expressed in terms of the viscosity and yield point of the printing fluid. Viscosity is generally defined as the printing fluid""s relative resistance to continual shear or flow. When the viscosity of a screen printing fluid is too low, excessive flow following printing causes poor image resolution. On the other hand, a printing fluid having too high a viscosity flows unevenly through the screen, resulting in poor transfer of the image or pattern to the target surface.
The yield point represents the printing fluid""s relative resistance to initial shear. Fluids having acceptable yield point for screen-printing possess high apparent viscosity when stationary, but readily flow when exposed to shear forces.
Part of the poor transfer of the image associated with high viscosity materials is due to xe2x80x9cstringingxe2x80x9d of the high viscosity materials. xe2x80x9cStringingxe2x80x9d is the visible strands of material as the screen pulls away from the substrate. Some adhesive compositions, particularly pressure sensitive adhesive compositions, are prone to stringing, which makes them undesirable for screen-printing. Stringing can be reduced or eliminated by controlling the molecular weights of the polymers and prepolymers in the composition. However, elimination of high molecular weight polymers from the composition may limit adhesive performance.
Attempts to modify adhesive and other coatings to meet these rheological requirements for screen printability have taken several routes. Addition of solvents or other carrier liquids to coating formulations has traditionally been one means for adjusting rheology of coating formulations. Although effective in providing screen printable compositions, use of solvents has several undesirable drawbacks, including potential for screen clogging due to solvent volatility and the need to recover and dispose of the solvent after printing. When water-based printable compositions are used, similar water removal and disposal process steps and equipment are required. Such drying steps can also limit the use of types of surfaces or substrates: thermally sensitive surfaces or substrates must often be avoided.
Screen printable formulations that can be cured after printing by exposure to actinic or other radiation sources eliminate some of the processing difficulties and concerns associated with solvent- or water-based compositions. Using this method, a polymerizable mixture of monomers, initiators, crosslinkers and other reactive components is combined with other coating additives and this mixture is screen printed onto a surface. Once printed, the image or pattern is irradiated and cured using a radiation source such as electron beam, ultraviolet light or gamma radiation.
Several approaches have been described to obtain radiation curable formulations having the requisite rheological properties for screen-printing. Co-reactive oligomers or polymers, such as oligomers or polymers having one or more functional groups that can copolymerize with monomeric components, can be added to tailor the viscosity of the reactive mixture. Similarly, nonfunctional oligomers or polymers can also be added to the mixture. With both of these approaches, the compatibility of these co-reactive and nonfunctional additives with both the unreacted mixture and the cured product can be a critical consideration. Lack of compatibility of these oligomeric or polymeric rheological additives can result in cured products having compromised appearance and physical properties.
Attempts have also been made to provide suitable viscosities for screen printable adhesive formulations by partially polymerizing the reactive mixture. Once the partially polymerized mixture is printed, the image or pattern is then exposed to radiation to complete the polymerization and cure.
However, incorporation of polymeric materials of too high molecular weight, either by direct addition or partial polymerization of the reactive mixture, may cause xe2x80x9cstringingxe2x80x9d. The weight average molecular weight of the polymers is typically below 1,000,000 and preferably below 600,000 to limit the elongation viscosity and result in less stringing of the adhesive during screen-printing. Limitation of molecular weight, however is not a complete solution, because such changes can diminish adhesive properties.
Addition of inorganic particulate fillers, such as silica, is another approach to controlling viscosity and yield point of radiation curable screen printable compositions. While providing suitable rheological characteristics, such inorganic particulate additives are difficult to evenly disperse in the coating and can lead to diminished weatherability and appearance of printed pattern or image. In addition, for adhesive applications, addition of silica or other inorganic particles can drastically influence the adhesive properties.
Swellable, elastomeric, acrylate or vinyl ester microspheres are typically the particulate suspension polymerization products of acrylate, methacrylate, and/or vinyl ester monomers and free radically polymerizable polar monomers. These polymerizations are performed in the presence of a variety of emulsifiers, surfactants, and stabilizers and/or under particular process conditions that induce the formation and prevent the agglomerations of the microspheres. Depending on the monomers used and processing additives or methods employed, these microspheres can be hollow (i.e., having at least one internal void or cavity) or solid (i.e., having no internal voids or cavities); tacky or tack-free; water or solvent dispersible; lightly or highly crosslinked; and a range of diameters (from about 0.5 to about 300 microns) and polymeric morphologies.
Such microspheres, particularly those having pressure-sensitive adhesive properties, are useful either when coated from aqueous suspensions or solvent dispersions in providing repositionable or removable adhesive coatings for a variety of articles and substrates. As screen printable adhesive coatings, these suspensions and dispersions suffer from the same drying and carrier liquid disposal drawbacks as the water- and solvent-borne printable coatings discussed above. Additionally, screen-printed adhesives coatings formed from such suspensions or dispersions could yield discontinuous coatings.
Microspheres have also been blended with film forming adhesive polymer matrix materials to form composites having a range of adhesive and other functional properties. Due to the typically high molecular weight and viscosity of these adhesive matrix materials, such blends of microspheres would possess unsuitable rheological properties that would result in uneven flow through the screen mesh and exhibit undesirable xe2x80x9cstringingxe2x80x9d when printed.
Microspheres have also been used as fillers in radiation curable high performance acrylic pressure-sensitive adhesive formulations. In such formulations, microspheres are combined with a free radically polymerizable monomer mixture. The mixture is then flood-coated onto a surface and, in an oxygen-free atmosphere, polymerized by exposure to heat or other radiation sources. The polymerized product is described as an interpenetrating pressure-sensitive adhesive polymer network wherein the polymerized monomer mixture forms an interpenetrating polymer network within the boundaries of the microspheres.
Briefly, in one aspect of the present invention, a contact printable adhesive composition is provided comprising (a) swellable, elastomeric microspheres and (b) at least one polymerizable monomer or a mixture of polymerizable monomers capable of swelling the microspheres. As used in this application xe2x80x9ccontact printable adhesive compositionxe2x80x9d is a composition that can be used in a contact printing process and then cured. The cured contact printable adhesive composition, that is, the polymerization product of component (a) and (b) above, will generally be referred to as xe2x80x9ccontact printed adhesivexe2x80x9d.
It has been discovered that swellable, elastomeric microspheres can be used as rheological modifiers to create contact printable adhesive compositions.
When the microspheres are combined with polymerizable monomer mixtures to provide compositions having the proper viscosity and yield point, these contact printable adhesive compositions flow sufficiently to provide a fairly smooth surface in a short amount of time after screen printing. Compositions are selected to provide a yield point that is high enough to maintain printing resolution after printing onto a substrate and/or provide sufficient adhesive thickness.
Optionally, viscosity modifiers such as thermoplastic polymers and copolymers, semi-crystalline polymers, and macromers may be added to the contact printable adhesive composition to modify the viscosity of and to alter adhesive performance.
In yet another embodiment, functional inorganic particles may be added to the contact printable adhesive composition. For example, an electrically conductive contact printable adhesive composition can be prepared by adding electrically conductive materials to the contact printable adhesive composition. In this application, the present invention also offers the further advantage of concentrating the conductive particles interjacent to the swollen microspheres. Fewer conductive particles are required when compared to non-microsphere containing conductive adhesive compositions because the microsphere domains take up a large amount of the space in the composition.
Another example of functional inorganic particles is the addition of luminescent particles. A luminescent contact printable adhesive composition can be prepared by adding luminescent particles to the contact printable adhesive composition.
In another embodiment, a method of using the contact printable adhesive composition is provided comprising the steps of: (a) providing a contact printable adhesive composition comprising: (i) swellable, elastomeric microspheres and (ii) at least one polymerizable monomer or a mixture of polymerizable monomers capable of swelling the microspheres, (b) applying the contact printable adhesive composition to a substrate using a suitable contact printing process.
This invention describes adhesive compositions that are applicable to contact printing methods. Examples of contact printing methods include lithography, gravure, flexography, and screen printing methods. Such printing methods can be used to provide adhesive-coated articles wherein the adhesive is actually printed on at least one major surface of a substrate. Such articles can include tape, flags, notes, sheets, signage, flexible circuits, or any another article than can be manufactured using conventional flood coating techniques. Uniquely, printable adhesives may provide an advantage in situations wherein flood coating may be an inappropriate xe2x80x9ccoatingxe2x80x9d method for an adhesive.
Surprisingly, such compositions possess a non-Newtonian rheological character that particularly suits screen printing equipment and processes. These microsphere-containing compositions and processes are substantially solvent free in nature and resist stringing and screen mesh clogging. While the adhesive compositions of the present invention are uniquely useful for contact printing, they may also be used in flood coating processes, although it is known that flood coatable adhesives are not necessarily useful as contact printable adhesives.
In a more preferred embodiment, the present invention provides a contact printable adhesive composition comprising: (a) about 2.5 to about 15% by weight, preferably about 5 to about 10% by weight, of swellable, elastomeric acrylate or vinyl ester microspheres; and (b) about 85 to about 97.5% by weight, preferably about 90 to about 95% by weight, of at least one polymerizable monomer or a mixture of polymerizable monomers capable of swelling the microspheres. The composition is substantially solvent-free and exhibits non-Newtonian rheological behavior and acts as a pseudoplastic. Compositions of the present invention have viscosity in the range of about 1 Pascal-second (Pas) to about 1000 Pascal-second at a shear rate of 1 secxe2x88x921 and a viscosity range of about 1 Pascal-second to about 100 Pascal-second at a shear rate of 100 secxe2x88x921 and a yield point of about 0.25 Pascal (P) to about 1000 Pascal.
Swellable, elastomeric microspheres useful in the present invention are preferably acrylate-based, pressure sensitive adhesive microspheres prepared by suspension polymerization processes. They may be solid, hollow or a mixture and have a volume average diameter of about 0.5 micrometers to about 300 micrometers, preferably about 1 to about 100 micrometers.
Polymerizable monomers and mixtures of such monomers useful in the present invention must be capable of swelling the swellable, elastomeric microspheres and can be selected with regard to the desired properties of the cured contact printed adhesive material. Depending on the components of the polymerizable monomer or resin mixture, contact printable coatings of the present invention exhibit either pressure-sensitive adhesive properties or heat-activatable adhesive properties.
For example, if pressure-sensitive adhesive contact printable formulations are desired, then polymerizable monomer mixtures comprising alkyl acrylate esters and monoethylenically unsaturated reinforcing comonomers wherein the polymerization product of the monomers and comonomers has a glass transition temperature of less than about xe2x88x9215xc2x0 C. are preferred. Heat-activatable adhesive contact printable compositions can be prepared from the same alkyl acrylate esters and higher proportions of monoethylenically unsaturated reinforcing monomers. The glass transition temperatures of such heat-activatable adhesive compositions are typically greater than 0xc2x0 C., although such temperatures could be less than 0xc2x0 C.
In this application:
xe2x80x9cswellablexe2x80x9d means a crosslinked polymeric material that swells in a reactive monomer mixture to an extent larger than its original dimension and forms a dispersion, substantially of individual particles;
xe2x80x9celastomericxe2x80x9d means amorphous or noncrystalline materials that can be stretched to at least twice their original length and that will retract rapidly to substantially (generally at least about 75% of, preferably, at least about 90% of) their original dimensions upon release of the force;
xe2x80x9csubstantially solvent freexe2x80x9d means a composition that has been prepared without the use of large amounts of solvent, that is, less than 5 percent by weight of the screen printable composition, preferably less than about 2 percent, and more preferably no additional solvent at all;
xe2x80x9csolventxe2x80x9d means conventional organic solvents generally used in the industry which include, for example, toluene, heptane, ethyl acetate, methyl ethyl ketone, acetone, and mixtures thereof;
xe2x80x9cpressure-sensitive adhesivexe2x80x9d means an adhesive material that is aggressively tacky at room temperature and firmly adheres to a variety of dissimilar surfaces upon mere contact without the need of more than finger pressure, yet has a sufficiently cohesive and elastic nature so that, despite its aggressive tackiness, it can be handled with the fingers and removed from smooth surfaces without leaving a residue (Pressure-Sensitive Tape Council Test Methods, 1985, p. 5);
xe2x80x9cheat-activatable adhesivexe2x80x9d means an adhesive material that firmly adheres upon application of heat or of pressure or both.